This invention relates to heat-recoverable articles.
Heat-recoverable articles, especially heat-shrinkable articles, are now widely used in many areas where insulation, sealing and encapsulation are required. Usually these articles recover, on heating, towards an original shape from which they have been previously deformed, but the term "heat-recoverable" as used herein also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed.
Heat-recoverable articles are typically made from polymeric materials exhibiting the property of plastic or elastic memory as described, for example, in U.S. Pat. Nos. 2,027,962, 3,086,242 and 3,957,382. In other articles, as described, for example, in British Pat. No. 1,440,524, an elastomeric member such as an outer tubular member is held in a stretched state by a second member, such as an inner tubular member, which, upon heating, weakens and thus allows the elastomeric member to recover.
Such heat-recoverable articles are frequently in the form of heat-shrinkable tubes, caps or boots or other articles having a closed cross-section and one or more open ends. One widely used method of making such articles is to mould a molten, crystalline, thermoplastic polymer into a shape which approximates to the shape of the final covering required; cross-link the shaped polymer; heat the cross-linked, shaped polymer to a temperature above its crystalline melting point; expand the hot, cross-linked, shaped polymer, e.g. by a mandrel or internal pressure; and cool the polymer in its expanded shape. The present invention especially relates to such heat-shrinkable articles, including in particular (but not limited to) those made by the process described above.
One serious limitation of such heat-shrinkable articles has been that if too high an expansion ratio (i.e. the ratio of the dimension of the article after expansion to the corresponding dimension before expansion) is used in making the article, the article tends to split on expansion or during shrinkage, and we have observed that this problem is particularly noticeable at the open ends of such articles. The problem is especially acute if the article has been cut so as to leave a notch or nick which may propagate on recovery and/or when the article is caused to recover about a large substrate leaving a high degree of unresolved recovery. In accordance with the present invention, we have discovered that it is advantageous if the or each open end of the article is formed by a section such that, when the article is fully shrunk, the open end is larger than the adjacent portion of the article. In particular we have found that the adjacent portion can then be given a higher expansion ratio than would otherwise be possible without danger of splitting.